Perhaps the most frequently-cited of the "quarrying " or "engineering" features at Rhosyfelin is the so-called "bluestone monolith extraction point" near the tip of the spur, and very close to sampling point number 8 as examined by Rob Ixer and Richard Bevins.
Well, I was down in the valley again yesterday, and research continues.
Well there is not really a recess here at all, and given the nature of the bedrock above and below MPP's right hand, it is vanishingly unlikely that a bluestone monolith with dimensions 2m x 20 cm x 20 cm could have survived physical extraction by quarrymen from here, since the rock is criss-crossed with deep fractures. As I mentioned in a previous post, there are at least ten intersecting fracture planes within a very small area. I'm amazed that the geologists did not point this out to Mike and his colleagues. We are talking rock mechanics and common sense here.
The other observation from my latest visit is that on the rock face from which the pillar is supposed to have been extracted, there is clear evidence that thin slabs of rhyolite have fractured and fallen away over a long period of time -- and not all on one occasion. If you just look at the edges of the fractures on the rock face, some are quite young, others are moderately weathered and others are very old (rounded off and heavily abraded). This little rock face has a complicated history, as any geologist or geomorphologist will confirm. On the illustrations pasted below, click to enlarge.